Polymer Electrolyte Membrane (PEM) fuel cells offer low weight and high power density and are being considered for automotive and stationary power applications. Current approaches for preparing a membrane electrode assembly (MEA) for PEM fuel cells can be broadly divided into two different categories: powder type and non-powder type. The powder type involves the process of catalyzation on a high surface area of carbon. The prepared carbon supported catalyst is mixed with binder and then applied to the membrane followed by gas diffusion layer (GDL) addition or to the GDL followed by membrane addition. A colloid mixture containing Pt/C powder, perfluorosulfonate ionomers (PFSI, such as Nafion) and solvent using ultrasonic treatment is prepared. This paste was then spread over the wet proofed gas diffusion layer of carbon paper. The electrodes with the paste on it were hot-pressed to both sides of a membrane to fabricate MEA. The observed increase in MEA performance was attributed to the increase in contact area between the PFSI and the Pt particles.
A catalyst decaling process in order to produce a dense and thin catalyst layer has also been developed. The first step in this process is preparing ink containing Pt/C powder, Nafion solution and solvent. This ink is then applied to a Teflon blank and heated until dry. More layers of Pt/C/Nafion ink are added until the desired catalyst loading is achieved. The catalyst coated Teflon blanks are hot pressed to the Nafion membrane. Then the Teflon blank is peeled away from the membrane, resulting in the MEA.
The catalyst layer in powder type MEA has a uniform concentration profile of the catalyst, since the Pt/C powder is thoroughly mixed with the binder before being applied to the membrane or GDL. A high content of Pt in the Pt/C powder allows reducing the thickness of the catalyst layer without sacrificing the catalyst loading per area of electrode. However, it is difficult to control the particle size of the catalyst when the Pt to carbon ratio increases more than 40 wt %.
In order to overcome this limitation, several non-powder type processes were developed. These processes create the catalyst directly on the surface of carbon electrode or membrane. A two-step impregnation-reduction method has been previously described. The Nafion membrane first undergoes an ion exchange reaction with a metal salt. Next, the impregnated membrane is exposed to a reducing agent to form a catalyst layer directly on the membrane. Another method is evaporative deposition, in which a Pt salt is evaporated and deposited on a membrane. A third MEA preparation technique is sputtering in which a very thin layer of sputter deposited platinum on a wet-proofed GDL performs very similarly to a standard E-TEK electrode. However, this technique is not a volume production method. It requires expensive vacuum equipment and cannot be used for fabrication of large structures with complex shapes.
A non-powder type electrodeposition technique has attracted attention due to its ease of preparation and low cost requirement. An electrochemical catalyzation (ECC) technique has been demonstrated to improve the utilization of Pt catalyst. In this technique platinum ions are diffused through a thin Nafion layer and electrodeposited only in regions of ionic and electronic conductivity. This post-catalyzation process can avert the loss of active Pt site by PTFE binder coverage. However, this process is strongly limited by diffusion of Pt complex ion across the Nafion layer. To avoid this limitation, carbon has been impregnated with H2PtCl6 and applied an electrochemical pulsed current to deposit Pt in the Nafion active layer. This process guarantees a smaller active layer thickness and high platinum mass fraction up to 40 wt. %. However, in terms of Pt concentration distribution, it has a profile like that of a powder type process, and Cl— ions produced from electrodeposition of Pt from H2PtCl6 remain in the active layer. The Cl— ions are known to poison platinum and reduce the catalytic activity of platinum. There is currently no technique to replace conventional powder type MEA preparation methods that will help achieve industry goals of reducing the cost and increasing the efficiency of polymer electrode membrane fuel cells.
Therefore, a polymer electrode membrane fuel cell that is capable of being produced at a reduced cost and has an increased operating efficiency is desirable.